The present invention relates to an insulated gate thin film transistor and, more particularly, to an insulated gate thin film transistor( referred to as "TFT" hereinafter) with an amorphous or microcrystalline semiconductor film.
FIG. 1 shows a sectional view of a conventional insulated gate thin film transistor(TFT).
The TFT of FIG. 1 comprises an insulating substrate 1, a gate electrode 2, a gate insulating film 3, a semiconductor film 4, a source electrode 5, a drain electrode 6.
The insulating substrate 1 is made of glass, ceramic, or crystal. The gate electrode 2 is made of Cr, Al, NiT or Au etc. The gate insulating film 3 is made of SiO, SiO.sub.2, Al.sub.2 O.sub.3, Ta.sub.2 O.sub.5, Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, or MgF.sub.2 etc. The semiconductor film 4 is composed of CdS, CdSe, Te, or PbS etc. The source electrode 5 and the drain electrode 6 are made of Al, Au, Ni, Cr or In etc,. so that they are connected to the semiconductor film 4 in an ohmic contact.
When the above-constructed TFT is applied for driving a liquid crystal panel in a multiplex driving, the following features must be presented by the TFT: showing a high OFF resistance (R.sub.off) to provide a sharp cutting operation; showing a low ON resistance R.sub.on to improve an ON-OFF ratio R.sub.off /R.sub.on ; having a high switching speed; and being stable even after over a long period of time.
To satisfy the above requirements, the gate insulating film 3 in the TFT should have the following features:
1. a high resistance, free of any pin holes, so that good reliability is expected along with the ability to withstand high voltages; PA1 2. a low mobile ion density; PA1 3. a low interface level with a semiconductor film; and PA1 4. a large field effect effect against the semiconductor film.
Although requirements 1 and 4 conflict with each other, so that the conventional TFT cannot satisfy all the requirements, this could be achieved by anodization.
For example, sputtering or CVD cannot provide a thin film made of SiO.sub.2, Si.sub.3 N.sub.4 etc. below a thickness of 2000-3000 .ANG., free of pin holes. However, anodization can provide an insulating film in a thickness of several hundreds of .ANG., free of pin holes, in which the insulating films can withstand a high voltage. When a voltage to be applied to the gate electrode is made constant, the field effect against the semiconductor film surface is proportional to the dielectric constants of the insulating films and inversely proportional to the thickness of the films. Thus, anodization can provide a thin film for providing a high field effect. Unfortunately, according to a conventional method, the anodized thin film may be damaged when a semiconductor film is disposed over the thin film, whereby the dielectric property of the thin film is degraded and is not suitable for the gate insulating film for the TFT. Since the preparation of the semiconductor film must occur after the preparation of the gate insulating film in view of the TFT structure, the degradation of the dielectric property cannot be avoided.